The present invention, in some embodiments thereof, relates to an apparatus and method for detecting myocardial ischemia using analysis of high frequency components of an electrocardiogram and/or of a cardiac electrogram, and, more particularly, but not exclusively, to an implantable such apparatus and method.
An electrocardiogram (ECG) is used to measure rate and regularity of heartbeats, as well as a size and position of heart chambers, presence of damage to the heart, and effects of drugs or devices used to regulate the heart.
Usually two or more electrodes are used for electrocardiogram (ECG) measurement. The electrodes can be combined into a number of pairs. Output from a pair of electrodes is known as a lead.
An ECG is a common way to measure and diagnose abnormalities in electrical activity of the cardiac muscle and abnormal rhythms of the heart, particularly abnormalities caused by damage to conductive tissue that carries electrical signals, or abnormal rhythms caused by electrolyte imbalances. In a condition of myocardial infarction (MI), the ECG can identify if the heart muscle has been damaged and sometime also indicate the location of damage, though not all areas of the heart are covered.
A typical ECG device detects and amplifies tiny electrical changes on a subject's skin which are caused when a heart muscle depolarizes and subsequently repolarizes during each heartbeat. At rest, each cardiac muscle cell is negatively charged, causing a membrane potential across its cell membrane. A cell's activation phase commences with depolarization, initiated by an influx of positive cations, Na+ and Ca++, and decreasing the absolute value of the negative charge towards zero. The depolarization activates mechanical mechanisms in the cardiac muscle cell which causes contraction in the cardiac muscle. During each heart cycle, a healthy heart has an orderly progression as a wave of depolarisation which is triggered by cells in the sinoatrial node spreads out through the atrium, then passes through the atrioventricular node and finally spreads over the ventricles. The progression is detected as waveforms in the recorded potential difference (or voltage) between electrodes placed on either side of the heart and may be displayed as a graph either on screen or on paper. The produced signal reflects the electrical activity of the heart, and different leads express more clearly different parts of the heart muscle.
A typical ECG trace of the cardiac cycle (heartbeat) consists of a P wave, a QRS complex, a T wave, and a U wave which is normally visible in 50% to 75% of ECG traces. A baseline voltage of the electrocardiogram is known as the isoelectric line. Typically, the isoelectric line is measured as the portion of the ECG trace following the T wave and preceding the next P wave.
A standard ECG traces usually filters out high frequency (HF) components, typically above 100 Hz. In some commercial implementations, lower thresholds such as 75 Hz or even 50 Hz are used for the low-pass filtering process. In general, the noise level is such that high frequency components, above 150 Hz, which are typically measured in micro-volts, are not reliably isolated from a single ECG trace and identified or measured. In order to measure and process high frequency components, one typically needs to use signal-to-noise enhancement schemes such as filtering and averaging.
An article by George B. Moody, Roger G. Mark, Andrea Zoccola and Sara Mantero titled “Derivation of Respiratory Signals from Multi-lead ECGs”, published in Computers in Cardiology 1985, vol. 12, pp. 113-116, Washington, D.C.: IEEE Computer Society Press, describes a signal-processing technique which derives respiratory waveforms from ordinary ECGs, permitting detection of respiratory efforts.
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The disclosures of all references mentioned above and throughout the present specification, as well as the disclosures of all references mentioned in those references, are hereby incorporated herein by reference.